How can I rid lilac bushes of the white substance that has appeared on nearly every leaf?
One of the more common fungus diseases of lilac is powdery mildew, which covers the leaves with the whitish, felt-like patches of mycelium that you have observed. It usually occurs late in the growing season in the St. Louis area. It’s a rather superficial disease which requires no chemical treatment. Rake up and dispose of fallen leaves. Good sanitation minimizes infections in subsequent seasons.
